Within the framework of steel metallurgy, long products are made from semi-finished products such as blooms (cross-section usually from 125 to 400 mm2) and billets (cross-section usually from 50 to 125 mm2) by casting with a continuous caster or rolling at a blooming mill.
As used herein throughout this application, long products are products with one dimension (length) being at least 10 times higher than the two other dimensions (as opposed to flat products) and include bars, rods, wires (coiled or not, for making e.g. bolts and fences), structural shapes and sections, rails, pipes, and tubes, e.g. for use in civil construction, mechanical engineering, energy, transport (railway, tramway), household and furniture. Bars are long products with square, rectangular, flat, round, or polygonal cross sections. Rounds can reach a diameter of about 250 mm. They are sometimes cold-drawn or even ground to very precise dimensions for use in machine parts. A special group of rounds are the reinforcing bars. Produced in diameters from about 10 to 75 mm, they provide tensile strength to concrete sections subjected to a bending load. They normally have hot-rolled protrusions on their surface to improve bonding with concrete.
Hot-rolled wire rods are produced in diameters between about 5 and 15 mm and may be shipped in coils. Rods may be cold-drawn into wires which may be afterwards covered by a coating for corrosion protection. The use of wire is extremely wide, ranging from cords for belted tires to cables for suspension bridges.
The most common structural shapes are wide flange I-beams, H-beams, L-beams, and T-beams. Such shapes are standardized, and may include railroad rails and special rails, e.g. for cranes and heavy transfer cars or for use in mines and construction.
Tubular steel long products may be broadly grouped into welded and seamless products. Longitudinally welded tubes are normally produced up to about 500 mm in diameter and/or about 10 mm in wall thickness. Pipes produced from heavy plates are also longitudinally welded after being formed and can be from about 0.5 m to 2 m in diameter, with a wall thickness up to about 180 mm. Seamless tubes are usually subjected to more demanding service; and may be rolled in diameters ranging from 120 to 400 mm and/or in a wall thickness up to about 15 mm, although special rolling mills can often increase their diameter to 650 mm. Smaller diameter tubes, both welded and seamless, can be produced by reduction mills or cold-drawing benches. Tubes are frequently machined on both ends for various coupling systems and coated with organic material.
The importance of providing protection against corrosion for ferrous (e.g. iron or steel) long products used under harsh environmental conditions, e.g. outdoors, is well known. Coating a ferrous material (mainly steel) with zinc is a very effective and economical means for accomplishing this goal. Zinc coatings are commonly applied by dipping or passing the steel long product to be coated through a molten bath of the metal. This operation is termed “galvanizing”, “hot galvanizing” or “hot-dip galvanizing” (HDG) to distinguish it from zinc electroplating processes. In this process, a solidified layer of zinc is formed on the product surface and the zinc coating layer formed as a result is strongly adhered to the surface of the article by an iron/zinc intermetallic alloy which forms during the galvanizing process. It is well known that oxides and other foreign materials (“soil”) on the surface of the steel article interfere with the chemistry of the galvanizing process and prevent formation of a uniform, continuous, void-free coating. Accordingly, various techniques and combinations of techniques have been adopted in industry to reduce, eliminate, or at least accommodate, oxides and soil as much as possible.
Improvement in the properties of galvanized steel products can be achieved by alloying zinc with aluminum, and optionally magnesium. For instance addition of 5 wt. % aluminum produces an alloy with a lower melting temperature (eutectic point at 381° C.) which exhibits improved drainage properties relative to pure zinc. Moreover, galvanized coatings produced from this zinc-aluminum alloy (known as Galfan, subject to standard specifications such as ASTM B 750-99, ASTM A 856-98) have greater corrosion resistance, improved formability and better paintability in comparison to a conventional galvanized coating, i.e. formed from pure zinc. Galfan coatings advantageously combine the passive corrosion inhibition of aluminum oxidation with the active and passive effects of zinc. Galfan-coated wires may be drawn (subject to standard specification ASTM A 764) into spring wires, strands (standard specification ASTM A 855), chain link fences (standard specifications ASTM A 817-94 and ASTM A 824-95), gabions (standard specification ASTM A 974-97), and steel-reinforced aluminum conductors (standard specifications ASTM B 232-99 and ASTM B 401-99). Further advantages of Galfan coated wires, vis-à-vis conventional galvanized wire, have been evidenced for steel springs, including consistency of spring length (associated with a decreased frictional interaction with coiling tools), and good adherence of the Galfan coating to organic coatings. However, zinc-aluminum galvanizing is known to be particularly sensitive to surface cleanliness, so that various difficulties, such as insufficient steel surface wetting and the like, are often encountered when zinc-aluminum alloys are used in galvanizing.
Many techniques and combinations thereof have been adopted in industry to reduce, eliminate, or at least accommodate, oxides and soil as much as possible. In essentially all these processes, organic soil, that is, oil, grease, rust preventive compounds, is first removed by contacting the surface to be coated with an alkaline aqueous wash (alkaline cleaning). This may be accompanied by additional techniques such as brush scrubbing, ultrasound treatment and/or electro-cleaning, if desired. Then follows rinsing with water, contacting the surface with an acidic aqueous wash for removing iron fines and oxides (pickling), and finally rinsing with water again. All these cleaning-pickling-rinsing procedures are common for most galvanizing techniques and are industrially carried out more or less accurately.
Another pre-treatment method used for high strength steels, steels with high carbon contents, cast iron and cast steels is a mechanical cleaning method called blasting. In this method, rust and dirt are removed from the steel or iron surface by projecting small shots and grits onto this surface. Depending on the shape, size and thickness of the parts to be treated, different blasting machines are used such as a tumble blasting machine for bolts, a tunnel blasting machine for automotive parts, etc.
There are two main galvanizing techniques used on cleaned metal (e.g. iron or steel) parts: (1) the fluxing method, and (2) the annealing furnace method.
The first galvanizing technique, i.e. the fluxing method, may itself be divided into two categories, the dry fluxing method and the wet fluxing method.
The dry fluxing method, which may be used in combination with one or more of the above cleaning, pickling, rinsing or blasting procedures, creates a salt layer on the ferrous metal surface by dipping the metal part into an aqueous bath containing chloride salts, called a “pre-flux”. Afterwards, this layer is dried prior to the galvanizing operation, thus protecting the steel surface from re-oxidation until its entrance in a molten zinc bath. Such pre-fluxes normally comprise aqueous zinc chloride and optionally contain ammonium chloride, the presence of which has been found to improve wettability of the article surface by molten zinc and thereby promote formation of a uniform, continuous, void-free coating.
The concept of wet fluxing is to cover the galvanizing bath with a top flux also typically comprising zinc chloride, and usually ammonium chloride, but in this case these salts are molten and are floating on the top of the galvanizing bath. The purpose of a top flux, like a pre-flux, is to supply zinc chloride and preferably ammonium chloride to the system to aid wettability during galvanizing. In this case, all surface oxides and soil which are left after cleaning-pickling-rinsing are removed when the steel part passes through the top flux layer and is dipped into the galvanizing kettle. Wet fluxing has several disadvantages such as, consuming much more zinc than dry fluxing, producing much more fumes, etc. Therefore, the majority of galvanizing plants today have switched their process to the dry fluxing method.
Below is a summary of the annealing furnace method. In continuous processes using zinc or zinc-aluminum or zinc-aluminum-magnesium alloys as the galvanizing medium, annealing is done under a reducing atmosphere such as a mixture of nitrogen and hydrogen gas. This not only eliminates re-oxidation of previously cleaned, pickled and rinsed surfaces but, also actually removes any residual surface oxides and soil that might still be present. The majority of steel coils are today galvanized according to this technology. A very important requirement is that the coil is leaving the annealing furnace by continuously going directly into the molten zinc without any contact with air. However this requirement makes it extremely difficult to use this technology for shaped parts, or for steel wire since wires break too often and the annealing furnace method does not allow discontinuity.
Another technique used for producing zinc-aluminum galvanized coatings comprises electro-coating the steel articles with a thin (i.e. 0.5-0.7 μm) layer of zinc (hereafter “pre-layer”), drying in a furnace with an air atmosphere and then dipping the pre-coated article into the galvanizing kettle. This is widely used for hot-dip coating of steel tubing in continuous lines and to a lesser extent for the production of steel strip. Although this does not require processing under reducing atmospheres, it is disadvantageous because an additional metal-coating step required.
Galvanizing is practiced either in batch operation or continuously. Continuous operation is suitably practiced on steel long products such as wires, tubes, rods and rails. In continuous operation, transfer of the articles between successive treatments steps is very fast and done continuously and automatically, with operating personnel being present to monitor operations and fix problems if they occur. Production volumes in continuous operations are high. In a continuous galvanizing line involving use of an aqueous pre-flux followed by drying in a furnace, the time elapsing between removal of the article from the pre-flux tank and dipping into the galvanizing bath is usually about 10 to 60 seconds.
There is a need to combine good formability with enhanced corrosion protection of the ferrous metal article. However, before a zinc-based alloy coating with high amounts of aluminum (and optionally magnesium) can be introduced into the general galvanizing industry, the following difficulties have to be overcome:                zinc alloys with high aluminum contents can hardly be produced using the standard zinc-ammonium chloride flux. Fluxes with metallic Cu or Bi deposits have been proposed earlier, but the possibility of copper or bismuth leaching into the zinc bath is not attractive. Thus, better fluxes are needed.        high-aluminum content alloys tend to form outbursts of zinc-iron intermetallic alloy which are detrimental at a later stage in the galvanization. This phenomenon leads to very thick, uncontrolled and rough coatings. Control of outbursts is absolutely essential.        wettability issues were previously reported in Zn—Al alloys with high-aluminum content, possibly due to a higher surface tension than pure zinc. Hence bare spots due to a poor wetting of steel are easily formed, and hence a need to lower the surface tension of the melt.        a poor control of coating thickness was reported. in Zn—Al alloys with high-aluminum content, possibly depending upon parameters such as temperature, flux composition, dipping time, steel quality, etc.        
Thus a lot of technical problems remain to be solved in the steel galvanizing industry. Furthermore there are also problems which are specific to the galvanization of steel long products. Molten Galfan alloy is not compatible with most flux systems conventionally used in galvanizing. This limitation has led to wide usage of “double dipping” processes wherein the Galfan hot dip follows a conventional hot dip. For the proper galvanization of steel wires with a zinc-aluminum or zinc-aluminum-magnesium alloy, it is thus usually necessary to rely on the so-called double-dip technology, i.e. first dipping the steel long product into a zinc bath, and then dipping the zinc-coated steel wire into a second zinc-aluminum or zinc-aluminum-magnesium alloy bath. In this double dip processing the properly annealed, cleaned and fluxed steel acquires a galvanized coating in the first bath This coating will generally include a series of iron-zinc intermetallic compounds at the iron-zinc interface, together with an overlay that is nearly pure zinc. The series of iron-zinc intermetallic compounds can be a source of coating brittleness. When the galvanized steel long product enters the second bath containing molten Galfan, the bath temperature will generally be high enough to melt or dissolve the essentially zinc galvanized overlay and transform the iron-zinc intermetallic layer into an aluminum-iron-zinc intermetallic. Upon emergence from the Galfan bath a layer of essentially Galfan alloy solidifies on top of the transformed aluminum-iron-zinc intermetallic layer. Aluminum that enters into the aluminum-iron-zinc intermetallic layer inherently lowers the aluminum concentration in the second bath. Thus double dip processing requires precise monitoring and management of the aluminum concentration.
Such a double dip processing appears for instance in EP 1.158.069 disclosing a plated steel wire wherein the average composition of the plating alloy used in the second stage contains 4-20 wt. % Al, 0.8-5 wt. % Mg and the balance Zn, and wherein an Fe—Zn alloy layer of no greater than 20 μm thickness is present at the plating-base metal interface. Such wire coating double dip procedure suffers from many technical and economical disadvantages as follows:                the need to invest into two separate zinc-based baths,        a higher energetic consumption than with a single bath procedure since wires need to be heated twice, and be quickly cooled down in between the two process stages,        the difficulty and extra cost to maintain the aluminum content (and optionally the magnesium content) constant in the second zinc-based bath, as reported for instance by Frank Goodwin and Roger Wright in The process metallurgy of zinc-coated steel wire and Galfan bath management jointly published by International Lead Zinc Research Organization Inc (North Carolina, U.S.A) and Rensselaer Polytechnic Institute (Troy, N.Y., U.S.A).        a higher residence time of wires at high temperature than with a single bath procedure and consequently a higher loss of mechanical resistance (tensile strength).        
WO 03/057940 discloses a process for the preparation of a steel surface for hot-dip galvanizing in an aluminum-rich zinc-based (e.g. Galfan) molten bath, comprising the steps consisting of electrocleaning, ultrasonic cleaning or mechanical brush cleaning the surface, pickling the surface, and applying a protective layer to the surface by immersion in a flux solution, characterized in that cleaning is performed so as to obtain less than 0.6 μg/cm2 residual dirt, and the flux solution comprises a soluble bismuth compound. Although a bismuth-containing flux composition may provide good Galfan coating at speeds which are compatible with a continuous production line for the galvanization of wires, it also suffers significant disadvantages such as very restrictive conditions of the previous cleaning or pickling steps. WO 03/057940 also teaches that coating quality significantly decreases when the aluminum content in the zinc-based galvanization bath increases, and further experiments have shown that this technology becomes hardly practicable when the aluminum content in the zinc-based galvanization bath exceeds 5 wt. % and/or when the zinc-based galvanization bath further includes magnesium.
It is known in the art that the addition of magnesium to an aluminum-rich zinc-based galvanization bath enhances the corrosion resistance, especially in a saline environment, and that this beneficial effect is greater when the magnesium concentration increases. However it is also known in the art that magnesium addition in a zinc alloy bath may decrease the cracking resistance of the coating being formed. The main factor for this phenomenon appears to be the formation of an intermetallic compound MgZn2, the cracking resistance of which is low under the influence of mechanical stress. Furthermore magnesium addition in a zinc alloy bath leads to the formation of a relatively rough coating microstructure. Stress repartition within the coating being formed is consequently less homogeneous, and more important stress may appear at the interface of the different metallic phases constituting the coating. Thus, not only magnesium addition improves corrosion resistance at the expense of some manufacturing problems and of the coating quality, but also it tends to increase the formation of soil or dross which float at the surface of the zinc bath, as evidenced for instance in FIG. 1 of European patent No. 1.158.069.
WO 2011/009999 solves the above problems of magnesium addition by providing a coated long product, in particular a steel wire, by dipping it into a zinc alloy bath including 4-8 wt. % aluminum and 0.2-0.7 wt. % magnesium and, upon exit from said bath, cooling the coated product, wherein said cooling is controlled to impart to said metal coating a homogeneous microstructure having more than 25% by volume of a beta phase portion being responsible for a good ductility of the coating layer.
WO 02/42512 describes a flux for hot dip galvanization comprising 60-80 wt. % zinc chloride; 7-20 wt. % ammonium chloride; 2-20 wt. % of at least one alkali or alkaline earth metal salt; 0.1-5 wt. % of a least one of NiCl2, CoCl2 and MnCl2; and 0.1-1.5 wt. % of at least one of PbCl2, SnCl2, SbCl3 and BiCl3. Preferably this flux comprises 6 wt. % NaCl and 2 wt. % KCl. Examples 1-3 teach flux compositions comprising 0.7-1 wt. % lead chloride.
WO 2007/146161 describes a method of galvanizing with a molten zinc-alloy comprising the steps of (1) immersing a ferrous material to be coated in a flux bath in an independent vessel thereby creating a flux coated ferrous material, and (2) thereafter immersing the flux coated ferrous material in a molten zinc-aluminum alloy bath in a separate vessel to be coated with a zinc-aluminum alloy layer, wherein the molten zinc-aluminum alloy comprises 10-40 wt. % aluminum, at least 0.2 wt. % silicon, and the balance being zinc and optionally comprising one or more additional elements selected from the group consisting of magnesium and a rare earth element. In step (1), the flux bath may comprise from 10-40 wt. % zinc chloride, 1-15 wt. % ammonium chloride, 1-15 wt. % of an alkali metal chloride, a surfactant and an acidic component such that the flux has a final pH of 1.5 or less. In another embodiment of step (1), the flux bath may be as defined in WO 02/42512.
JP 2001/049414 describes producing a hot-dip Zn—Mg—Al base alloy coated steel sheet excellent in corrosion resistance by hot-dipping in a flux containing 61-80 wt. % zinc chloride, 5-20 wt. % ammonium chloride, 5-15 wt. % of one or more chloride, fluoride or silicafluoride of alkali or an alkaline earth metal, and 0.01-5 wt. % of one or more chlorides of Sn, Pb, In, TI, Sb or Bi. More specifically, table 1 of JP 2001/049414 discloses various flux compositions with a KCl/NaCl weight ratio ranging from 0.38 to 0.60 which, when applied to a steel sheet in a molten alloy bath comprising 0.05-7 wt. % Mg, 0.01-20 wt. % Al and the balance being zinc, provide a good plating ability, no pin hole, no dross, and flat. By contrast, table 1 of JP 2001/049414 discloses a flux composition with a KCl/NaCl weight ratio of 1.0 which, when applied to a steel sheet in a molten alloy bath comprising 1 wt. % Mg, 5 wt. % Al and the balance being zinc, provides a poor plating ability, pin hole defect, some dross, and poorly flat.
Although the methods described in the above documents have brought some improvements over the previous state of the art, they have still not resolved most of the technical problems outlined hereinbefore, especially the numerous problems associated with the double dipping processing, with respect to the galvanization of steel long products such as, but not limited to, wires, rods, bars, rails, tubes, structural shapes and the like.
Consequently there is still a need in the art for improving continuous processing conditions vis-à-vis the current double dip technique of galvanizing wires, as well as fluxing compositions used therefore.